I'm standing in the middle of a weed-strewn wasteland, looking for the sculpture park that's part of the Berlin Biennial. In the distance, some overdressed people are stumbling around a patch of ground that was once the "death strip" beside the Berlin Wall. Music wafts from a mound of rubble. Someone is peering intently at a birch tree. It has a label - this must be the art.

Dotted about the place, various other works resemble abandoned billboards, bus shelters and shanty dwellings. A series of holes decorate one patch of earth, as if excavated with a giant ice-cream scoop. This miserable patch of churned, fallow ground in the centre of Berlin has been squabbled over by developers ever since unification; it is a place with a haunted past and a contested present.

Among the broken lumps of masonry and rubbish is a shed in which a film by Lars Laumann tells the story of a Swedish woman who fell in love with the Berlin Wall and now believes they are husband and wife. In the dark, my jaw dropped. The story, I realised, is not a spoof. Eija-Riitta Berliner-Mauer really is Mrs Berlin Wall, and lives with her now retired husband, in the form of various small barbed-wire-topped models of himself, in a village in northern Sweden. She says the day the wall came down was an absolute disaster, but she loves her wall just the same. As well as her beloved husband and numerous cats, she also keeps various scale-models of guillotines for company. What turns her on is parallel lines, rectangular shapes, forms that divide (such as walls), and others that connect (such as bridges). Don't ask about the guillotines. She says she's an object-sexualist, and believes that objects have souls, feelings, desires and thoughts they share with her telepathically. Which isn't all that different from the art critic who also believes that man-made objects can talk and hold secrets they can share. Admitting you're an art lover might say more about you than you think.

Elsewhere, inside Ludwig Mies van der Rohe's 1968 Neue Nationalgalerie, a group of steel sculptures painted yellow are serving as a cloakroom. The welded sheets are draped with gallery-goers' coats, bags, crash helmets and packages. Gabriel Kuri's Items in Care of Items are abstract modernist sculptures improved by life, the severe forms animated by all the things being deposited and retrieved.

Hanging in the entrance, Paula Pivi's spiky decorative screen is a sort of gigantic rhinestone-encrusted portcullis, whose bling aesthetic is matched only by its title: If You Like It, Thank You. If You Don't Like It, I'm Sorry. Enjoy Anyway. We might take this as an equivalent to the phrase inscribed on the gates of hell, in Dante's Divine Comedy: Abandon all hope, ye who enter here. As it is, it's difficult to forget the ghost of Mark Wallinger's Sleeper, who spent night after night wandering this same space, pondering the conundrums of Berlin's history in a bear suit.

In a small booth, a 16mm film by Paul Sietsema provides a kind of lecture, as silent images show us old coins, frayed rope, pottery shards, crackled ceramic glaze, fragments of netting. Nothing is explained; it is as if the artist is inviting us to draw comparisons between these details, in a game of show-and-tell - but where the explanations are withheld.

The Neue Nationalgalerie section is the most consistent part of this biennial. It works as one large conversation piece. In the midst of it, Susan Hiller amplifies archived voices speaking in near-extinct languages, translations of which appear on a screen slung from the ceiling. It is a cinema of the unseen and the no-longer-heard. There are snatches of song and folk myth, exhortations and lamentations, last voices no one can understand speaking languages in which no new thing will ever again be said. The voices echo and mingle with all the other voices, objects, films and images in this gigantic room. The overall atmosphere is of light, transparency and sprawling drapery.

After the success of the last Berlin Biennial - called Of Mice and Men, and with a curatorial team led by artist Maurizio Cattelan - I was nervous that this latest, called When Things Cast No Shadow, would disappoint. It is less spectacular; there are fewer big-name artists. It proceeds by stealth, offering possibilities rather than answers, conditions rather than a theme. What is meant by the title? If art casts no shadow, it is either dead or without substance. More likely, the curators, Adam Szymczyk and Elena Filipovic, wish us to cast our own shadows here. The result is a more generous and rewarding biennial than most.

In the cellar at the KW Institute for Contemporary Art, several Belgians are staring at a small model of a black frigate. Then they stare boggle-eyed at one another and adopt strangely stiff and ludicrous postures for the camera. The one in the beard doesn't fool me. It's false. And the one in the plus-fours is a woman. Bursts of dramatic but arbitrary organ music alternate with dead silence. All this is incomprehensible, but hilarious.

To reach Jos de Gruyter and Harald Thys's film, The Frigate, one has to pass through the building's main hall, on whose floor Ahmet Öğüt has deposited several tons of well-laid black Tarmac, like a wall-to-wall carpet. Soon, the work suggests, the whole of Öğüt's native Turkey will be covered with the stuff. Why stop there? The Berlin sculpture park could do with a few truckloads. The hall looks like a parking lot, an arena waiting for something to happen. People pass through without a second's thought. This is not so much art that casts no shadow, as art that is all shadow.

There are other things here that eat the hours. Babette Mangolte's installation includes lovely black-and-white photos taken on a beach, film of fields and foliage, horse riders, gardens and interiors, and numerous choreographed fragmentary and inconsequential actions - the building and rebuilding of a wall of cigarette packages; a hand splayed on the back of a chair; photos of Manhattan shuffled on a table. I ask myself why all this apparently inconsequential material is so compelling. It is a matter of pace, cutting, focus, concentration and a sort of reserve.

Nearby, Michael Auder films himself smoking his last ever bag of heroin. He chases the dragon and drums his fingers on the table. Tomorrow, rehab. Auder's videos are shown in a red-lined, womb-like cabin. Maybe smack gives you a sense of safety, too. Auder undercuts this with his expert and rapid montages of war, death, eyes, mouths, ruins, guns and skulls. This is hardly the poet's laudanum reverie. An exhilarating film-maker, Auder doesn't flinch.

This somewhat voyeuristic theme - of the world consumed, the eye always hungry for one more image, whatever it might be - is continued in Kohei Yoshiyuki's famous 1970s black and white photographs of nightlife in a Tokyo park. The camera seeks out people cruising among the trees, couples having sex, voyeurs creeping up on them. We're there too, dogging the artist's camera. And, here in the gallery, we all watch each other looking, in an endless chain of hungry eyes.

But my favourite work is a film by Belgian Manon de Boer. Her camera records a live performance in Brussels of John Cage's silent work for piano, 4'33". The pianist sits and sets the timer. He sits and sits. Behind him, condensation fogs the window. We hear the ambient sound of distant traffic sizzle by on a wet road.

After four minutes and 33 seconds, the audience clap. As Cage's piece is performed a second time, the camera circles the room and we see the attentive audience. There is complete silence. You can almost feel the pressure in your ears. The camera wanders over to the window. Outside a winter gale is thrashing the bare trees, unheard. Silence, perhaps, casts its own shadows. And if you can marry walls that aren't there, who knows where the real darkness lies.